It is common commercial practice to store liquid materials in storage tanks. Typically, for many industrial applications, storage tanks will have a diameter from 100 to 300 feet and heights of 20 to 50 feet or more. The liquids stored in such storage tanks are diverse. For example, water or aqueous solutions of organic or inorganic chemicals may be stored in this manner, derivatives of agricultural products such as vegetable oils which are water soluble are likewise stored in this manner.
More commonly, however, large volume storage tanks of this nature are used in the production, collection and refining of crude oils and derivatives thereof such as crude oils containing naphthenic and aromatic components and refinery products such as gasolines, diesel fuels, jet fuels, fuel oils, kerosene, gas oils etc. and petrochemical derivatives such as benzene, xylenes, toluene, etc.
With the passage of time, solid materials, usually in finely divided form, will accumulate in the storage tank and settle at the bottom thereof. When the accumulation becomes excessive, it must be removed from the storage tank.
One manner in which this can be accomplished is to drain the tank and manually remove the sediments that are deposited therein. However, such a procedure is costly and time-consuming and can cause the workmen involved therein to be exposed to hazardous or potentially hazardous materials.
The problem of sediment accumulation is particularly accentuated insofar as the storage of crude oil and, in particular, aromatic and naphthenic crude oils is concerned. Such crude oils as introduced into the storage tank will normally contain aromatic, naphthenic and asphaltic components which are believed to be potentially reactive and/or condensable with each other. Moreover, a minor amount of water will normally be present in the crude oil (e.g., about 0.1 to 5 wt. %), but usually the water will not be present as a separate phase, but rather as small droplets of water emulsified to ionizable components of the crude oil, such as asphaltenes.
It is believed that molecular charge transfer forces, such as Van Der Waals forces, cause many of the molecular aromatics, naphthenic and asphaltic components of the crude oil to agglomerate and weakly bond to each other to form aggregates having a size sufficient to cause them to precipitate from the crude oil and settle at the bottom of a crude oil storage tank together with the emulsified water droplets to that the resultant "hydrocarbon sludge" will normally comprise highly aromatic components such as poly aromatic components in which a significant portion of the water (in the form of emulsified droplets) will be accumulated. Such sediment in the bottom of crude oil storage tanks is colloquially referred to as black sediment and water or hydrocarbon sludge or just plain sludge.
The hydrocarbon sludge that accumulates, as such, is of marginal economic value and, if manually removed usually represents a disposal problem.
It is known to remove sediments from a storage tank by agitating the liquid in the storage tank so as to resuspend the sediment so that a stream of sediment-containing liquid can be withdrawn from the storage tank and filtered as illustrated for example by Krajicek et al., U.S. Pat. No. 5,091,016.